Policy routing is a major topic on your BSCI exam, and you'll find quite a bit of policy routing going on in today's production networks. But what exactly is policy routing?
Laptop Battery Policy-based routing, generally referred to as "policy routing", is the use of route maps to determine the path a packet will take to get to its final destination. As you progress through your CCNP studies and go on to the CCIE (or to a Cisco Quality Of Service certification), you'll find that traffic can be "marked" by policy routing in order to give different levels of service to various classes of traffic. (This is done by marking the traffic and placing the different classes of traffic in different queues in the router, allowing the administrator to give some traffic higher priority for transmission.)
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Thinkpad There are some basic policy routing rules you should know:
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Microsoft Policy routing doesn't affect the destination of the packet, but does affect the path that is taken to get there.
Passing the BCSI exam and earning your CCNP certification requires you to know OSPF stub areas inside and out. Stub areas, total stub areas, stub stub areas ... and pretty soon your head is swimming. Then when you hear that EIGRP offers stub routing, your first reaction may be unprintable! But while EIGRP stub routing is effective in the right situation, it's not as complex as OSPF stub routing. Let's take a look at basic EIGRP stub routing.
Laptop Computers Policy routing can forward traffic based on the source IP address or the destination IP address (with the use of an extended ACL).
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Laptop Computer Policy routing can be configured at the interface level, or globally.
Occasionally, during your CCNA and CCNP studies, you'll run into a term that just doesn't quite make sense to you. (Okay, more than occasionally!) One such term is "reverse telnet". As a Cisco certification candidate, you know that telnet is simply a protocol that allows you to remotely connect to a networking device such as a router or switch. But what is "reverse telnet", CCNP home lab setup
Desktop Computer
Applying policy routing on an interface affects only packets arriving on that interface:
Notebooks R2(config)#int s0
Lenovo R2(config-if)#ip policy route-map CHANGE_NEXT_HOP
Hard Drive Applying the policy globally applies the route map to packets generated on the router, not on all packets received on all interfaces.
Travelstar Whether you're running policy routing at the interface level, on packets created locally, or both, always run the command show ip policy to make sure you've got the right route maps on the proper interfaces.
Gateway R2#show ip policy
Laptop Parts Interface Route map
Software local CHANGE_NEXT_HOP
Hard Drives Serial0 CHANGE_NEXT_HOP
Electronics And here's the big rule to remember....
Canon If a packet doesn't match any of the specific criteria in a route map, or does match a line that has an explicit deny statement, the data is sent to the routing process and will be processed normally. If you don't want to route packets that do not meet any route map criteria, the set command must be used to send those packets to the null0 interface. This set command should be the final set command in the route map.
Desktop Pc There are four possibilities for an incoming packet when route maps are in use. The following example illustrates all of them.
Desktop Computers R2(config)#access-list 29 permit host 20.1.1.1
Think Pad R2(config)#access-list 30 permit host 20.2.2.2
Repair R2(config)#access-list 31 permit host 20.3.3.3
Data Recovery R2(config)#access-list 32 permit host 20.4.4.4
Cisco R2(config)#route-map EXAMPLE permit 10
Keyboard R2(config-route-map)#match ip address 29
Monitor R2(config-route-map)#set ip next-hop 40.1.1.1
Desktop
R2(config-route-map)#route-map EXAMPLE permit 20
Infosys R2(config-route-map)#match ip address 30
Refurbished Laptops Assuming the route map has been applied to the router's ethernet0 interface, a packet sourced from 20.1.1.1 would meet the first line of the route map and have its next-hop IP address set to 40.1.1.1.
Wipro A packet sourced from 20.2.2.2 would match the next permit statement (sequence number 20). Since there is no action listed, this packet would return to the routing engine to undergo the normal routing procedure. All traffic that did not match these two addresses would also be routed normally - there would be no action taken by the route map.
Lap Top Perhaps we want to specifically block traffic sourced from 20.3.3.3 or 20.4.4.4. We can use multiple match statements in one single route map, and have packets matching those two addresses sent to the bit bucket - the interface null0.
Refurbished R2(config)#route-map EXAMPLE permit 30
Memory R2(config-route-map)#match ip address 31
Intel R2(config-route-map)#match ip address 32
As400 R2(config-route-map)#set ?
Averatec as-path Prepend string for a BGP AS-path attribute
Hardware automatic-tag Automatically compute TAG value
Dual Xeon comm-list set BGP community list (for deletion)
Storage community BGP community attribute
Seagate dampening Set BGP route flap dampening parameters
Computer Sales default Set default information
Computer Hardware extcommunity BGP extended community attribute
Printers interface Output interface
Technology ip IP specific information
Mainframe level Where to import route
Samsung local-preference BGP local preference path attribute
Computer Repair metric Metric value for destination routing protocol
Used Computers metric-type Type of metric for destination routing protocol
Network origin BGP origin code
Digital Cameras tag Tag value for destination routing protocol
Desktops weight BGP weight for routing table
Cognos R2(config-route-map)#set interface null0
Hosting Any traffic matching ACLs 31 or 32 will be sent to null0, resulting in its being discarded by the router. Any traffic that didn't match any of the route map statements will be returned to the routing engine for normal processing.
Netfinity Knowing policy routing and how to apply it are essential skills for passing the BSCI exam, earning your CCNP, and becoming more valuable in today's job market. Get some hands-on practice in a CCNA / CCNP home lab or rack rental to go along with learning the theory, and you'll be writing and applying policy routing in no time at all.
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